Video tutorials on installing internal and external bearing BB's:
http://www.spadout.com/iv/how-to-ins...ottom-bracket/
http://www.spadout.com/iv/how-to-install-an-external-bearing-bottom-bracket/

Not sure what the procedure would be to fix the threading on a ruined BB. I'm sure a decent LBS could do it, but I don't now if it'll be very cheap.